Art of coating zinc



Patented June 21, 19 -11,:

UNITED STATE alums 1 ART OF COATING ZlNfi Gerald G. ltomig, ElirinsPark, Pa, American Chemical Paint Company. l'a.,,a corporation ofApplication November so, less, sci-a No. masts comma.

This invention relates to the art of treating 7 surfaces of zinc or itsalloys in order to provide them with an adherent substantially insolublecoating of phosphates.

The principal objects of the invention are, first, to provide an articlehaving a surface of zinc or its alloys with an improved phosphaticcoating of greater uniformity in thickness and color and of greatersuitability for the reception and retention of a final finish of paint,varnish, lacquer, japanor other similar organic protective material;second, to provide an improved method for the production of sucharticles, and, third, to provide improved materials for use in 15producing such articles. The invention also contemplates the productionof a coating of the character described on articles of zinc' or itsalloys which will prevent certain deleterious effects which zincnormally thereto. a v

A further ob'iect of the invention is to produce a coating of morepleasing character and color.

Among the more detailed objects of the invention is the provision of aso-called balanced phosphatic treating solution which will coat'zinc orits alloys much more uniformly than has heretofore been possible, andwhich will rehold a final finish of paint or the like..

Stiiianother object is to produce coatings of the character described ata minimum expense and in a minimum of time.

The nature of the invention will be better understood and appreciated ifattention is first directed to certain facts already familiar to thoseskilled in this art.

For example, it has long been known that a upon the zinc then acquires aphosphatic coating.

The meaningof the term balan in this art should be'kept in mind and isex- 55 piained as follows. If the solution is to produce has uponorganic protectivemateriais applied suit in a coating better adapted toreceive and 1 "asused or its bier,

(Ci. lite-cl its coating in a reasonably short time, it should be nomore acid than is necessary to prevent the precipitation of insolublephosphates of the metals present until such time as it is allowed to actupon the attachable metal to be coated, 5 such as zinc. In other words,the solution should not contain an excess of hydrogen ion in order thatit may act with reasonable rapidity.

But in the process of Patent No. 1,221,046 already referred to, greatcare must be exercised in connection with the treatment of articleswhich are only thinly covered with zinc or its alloys, such, forexample, as galvanized iron, If too much acid or acid of too greatstrength is applied to the surface of such an article, so much zinc maybe required to balance" the solution that the entire zinc coatingon thearticle may be removed before the phosphatlc coatlug forms.

The art then discovered that. by partial neul I tralization of the freeacids in the solution before applying it to the metal it was possible toavoid this difliculty. This was accomplished by the addition of metalliczinc, zinc oxide, zinc carbonate, etc., by means of which the solutioncould be made initially nearly or entirely in balance", so that it couldbegin to coat the. zinc or its alloys as soon as it began to act uponthem.

In addition to the foregoing, the art also discovered that with such abalanced solution as described just above, an additional saving of timeand materials could be efiected if the solution were used dilute andhot, say somewhere in the neighborhood of from 120 F. to the boilingpoint. However, the coatings produced in this way are very light incolor, ranging from pure white to a light gray, and the coating actionof the solution is extremely sensitive to slight variations in thesurface undergoing treat- 4o ment. For instance, on an ordinary piece ofzincor a die-casting, some areas may bewell coated and others'may remainentirely bare, and on a piece of spangled galvanized iron thisirregularity of coating effect is especially notice-'- able andobjectionable. In the latter case certain portions of every spangleescape coating at ail, while other parts may be coated perfectly. Thepresent invention overcomes all of the diiliculties in prior practicereferred to above and, 5 briefly stated, it,involves the discovery thatfar more satisfactory coatings of the character described can beproduced by adding to the treating solutiomin addition to theingredients already certain compounds; soluble in the solution, whichwill yield therein the cations of nickel and/or cobalt, the process ofthe invention consisting in subjecting the zinc surface to the action ofa solution containing the negative ions of phosphoric acid as its majoranion constituent, zinc ions as its major cation constituent, thecations of nickel and/or cobalt as a minor ionic constituent, and alsothe nitrate anion as a minor constituent.

Before proceeding to a detailed description of specific ways in whichthe invention can be realized, I should like to point out that verysmall quantities of the cations of nickel and/or cobalt markedly improvethe evenness of the coating action without materially ailfecting thecoating time; that larger concentrations produce darker coatings andslightly increase the time necessary to complete the coating; and thatstill higher concentrations cause a considerable increase in the coatingtime with the production of a thinner and generally less desirablecoating of lighter color..

I am aware that metals less basic than iron have been added to phosphatecoating solutions in order to accelerate the action and to color theresulting coating. For example, copper has been used as an acceleratingagent in the production of phosphatic coatings on iron, but myexperience has shown that the addition of copper to a balanced solutioncontaining the ions of phosphoric acid, nitric acid and zinc, causes thedeposited coating to be black and loosely adherent. Very smallquantities of copper darken the coating somewhat and allow theproduction of adherent coatings, but such coatings have a greatlyreduced corrosion resistance, and organic finishes applied thereto failrapidly. The same observations apply for silver, mercury, antimony andother metals below hydrogen in the electromotive series.

Lead also has been proposed, but insofar as I am aware, this is the onlymetal which has been above hydrogen in the electromotive series. Themetals below hydrogen are deposited at least partially in the metallicstate and when present in the coating such metals, of which copper andsilver are probably the most typical, all tend to pro-, duce smudgy andnon-adherent coatings or else to impair corrosion resistance or paintliie. Lead is a metal which is so slowly. attachable by weak acids thatit also is deposited largely in the metallic state when present in aphosphatic coating bath for zinc, and it is admitted by all thosefamiliar with the art that lead cannot serve as a satisfactory coloringor accelerating agent.

In direct contrast to the foregoing, I have found that nickel and cobaltmay be added to the treating solutions in considerable amounts withoutcausing loosely adherent coatings and without impairing the corrosionresistance or paint preservative properties of the coating. In thepractice of my invention I have been unable to find any nickel or cobaltin any coatings produced on thesurface of zinc or its alloys by means ofbalanced" solutions containing the ions oi these metals, zinc,nitricacid and phosphoric acid, and a it is to this fact that I ascribethe excellent corrosion resistance and paint preservativecharacteristics of my improved coatings.

As an illustration of the application oi my process and solution in theproduction oi excellent coatings on galvanized iron, for instance, thefollowing example is given:

A concentrated stock solution is prepared accordance with the followingiormula:

in v

' equivalents of zinc is present.

than that of the nickel.

sinners Formula N0. 1

Zinc oxide lbs '1. 530 Nickel carbonate lbs 0. 248 Phosphoric acid, 75%gallons 0. 5000 Nitric acid, 38 Be do 0. 0487 Water, q. s. ad do 1 0000The zinc oxide and nickel carbonate are suspended in the water. Thenitric acid and phosphoric acids are then gradually added with con stantstirring. When all gassing has ceased, and the solution is clear, it isready for use.

Approximately 8% by volume of this stock solution is now dissolved inwater in a vat fitted with steam coils or other suitable heating means.The solution is preferably heated to approximately 160 F., and thegalvanized iron sheets are immersed therein.

Arather violent evolution of hydrogen gas occurs on the surface of thesheet, which ceases almost entirely inside of about thirty seconds. Thesheet is now removed. washed with clean water and dried in anyconvenient way, as by heating in a warm air oven. The finished sheetswill now be covered with an adherent, dark grey, smoothcoating ofexcellent corrosion resisting and paint preservative properties.-Certain types of zinc and zinc alloy surfaces are more easily coated bythis solution than others. In the case of certain die-castings, forinstance, the above solution may be applied at a somewhat highertemperature, not exceeding its boiling point. The solution may likewisebe used at lower temperatures, if a longer time is'available forcoating. 3

The solution of Formula No. 1 gives a dark even coating-of excellentsmoothness. If a coating of lighter color is required less nickel may beused in the formula. It will be noted that in this formula oneequivalent of nickel is present for each nine equivalents of zinc. Evenat a nickel concentration of one equivalent of nickel to equivalents oizinc the coating produced is much more uniform than when the nickel isabsent.

As the proportion of nickel is increased the coating becomes darker incolor until a ratio of approximately one equivalent of nickel to two Agreater proportion of nickel than this produces lighter colored andthinner coatings of less corrosion resistance and paint-preservativevalue. I

Cobalt as a constituent of solutions of this kind acts very similarly tonickel. ening effect of cobalt is slightly less pronounced It may besubstituted practically weight for weight for the nickel, as the twometals have nearly identical equivalent weights. The maximum darkeningefl'ect oi the cobalt on the coating is reached atsomewhat less than oneequivalent of cobalt to one equivalent oi zinc. However, no advantageseems to be ch- However, the darktained by using either nickel or cobaltin a pro- 1 portion greater than one equivalent of nickel or cobalt totwo equivalents of zinc.

It is to befnoted that, other conditions remaining the same, theaddition oi. either nickel or 00- This retarding degree withoutaffecting the action of the solution materially. 0.12 equivalent ofnitrate anion to one equivalent of combined metallic cations in thecoating solution is found to be ample; but an amount of nitrate up toone equivalent of total metallic cations may be present without doingharm. As little as 0.05 equivalent of nitrate anion per equivalent oftotal metal cations is still enough to produce good results, but thecoating of a very active zinc may reduce the nitrate in this case sothat it may have to be replenished more rapidly than the otheringredients.

The presence of foreign cations and anions is not particularly harmfulin my solutions, but no substance may be allowed to accumulate whichdisturbs seriously the ionic relationship of the solutions. Forinstance, moderate amounts of sodium, calcium, or other'cations ofhighly basic metals may be present without harm: similar- "ly moderateamounts of sulphate, borate, chloride, etc., are not particularlyobjectionable. Thus one may prepare a solution similar to Formula No. 1except that cobalt takes the place of nickel, and that the nitrate andcobalt ions are added as sodium nitrate and cobalt sulphaterespectively.

Such a solution is given below:

Formula No. 2 Zinc oxide lbs 1.500 Phosphoric acid, 'l5% gallons 0.500Sodium nitrate L lbs 0.402 Cobalt sulphate, COs04.7H20 lbs 0.587 Waterq. s. ad gallons 1.000

This solution can be prepared by suspending the zinc oxide in the water,stirring in the phosphoric acid until clear, and then dissolving thereinthe cobalt sulphate and sodium nitrate.

This solution may be diluted with water to a 5 to strength by volume'andused hot, like the diluted Formula No. l, to produce coatings on zincbase die castings, galvanized iron, etc. However, replenishment of thecoating solution as it becomes depleted in strength may not beaccomplished indefinitely with Formula No. 2. If this is attempted therewill ultimately accumulate in the solution too great a quantity ofsodium and sulphate ions.

It is obvious, therefore, that in order to avoid the necessity for moreor less frequently discarding a solution which is continually used andreplenished, one should supply the requisite ions as free as possiblefrom foreign ions which may accumulate therein.

It has been discovered that the relative as well as the absolute rate ofconsumption of the constituents of my solutions in the coating operationvaries somewhat with the composition and surface condition of the metalbeing coated. Thus the relative rates of depletion of zinc, nickel, andnitrate ions for example, may vary somewhat under various conditions.expedient in order to maintain a constant time and color of coating, to'replenish'the various ingredients in a continually reused solution, bymeans of separate additions of different materials, each bearing apreponderance of one or the other of the necessary ions. For instance,one may employ, as replenishing agents, as required, nickel nitrate, andzinc phosphate or concentrated solutions of these.

As stated above, solutions of the character described maybe used to coatsurfaces of zinc or to at least 0.40 equivalent temperature, nature ofthe metal, etc.

It may, therefore, be 1 its alloys in the cold. This procedure becomesespecially useful, for example in coating surfaces of galvanized ironforming portions of. structures already erected, such as buildings,fences, etc.

In coating such structures one may use solu-' tions such as theconcentrate of Formula No. 1 diluted with water to, say, 20% strength,by volume. However, it may be found more convenient. to use a solutionespecially prepared for the purpose.

A solution especially well adapted for use in coating such surfaces inthe cold, especially when the'solution, is to be applied in a thin filmas by brushing, spraying, flowing, etc., has the follow- "ingcomposition:

Formula N0. 3

them withreasonable rapidity.

Zinc oxide lbs. 0.300 Phosphoric acid, 75% gallons 0.140 Nickel nitrateNi(NOa)2.6H2O lbs. 0.144 Isopropyl naphthalene sulfonic acid do 0.010Methanol gallons 0.100 Water, to make do 1.000

The solution is simply appliedto the surface to be coated and allowed toremain until visible action has ceased, which may take from five minutesto half an hour, depending on the prevailing face, after thoroughrinsing with clean water, is allowed to dry, when it is ready forpainting, etc.

It will be understood that the examples given are offered only by way ofillustration, and are not to be taken as limiting the scope of theinvention. One skilled in the art will readily appreciate that manyvariations are possible in the formulation of my solutions and in theirmode of application, as well as in the products resulting from theapplication of the process and the solutions.

I- claim:

1. A solution for coating zinc or its alloys comprising the anions ofphosphoric and nitric acids and the cations of zinc and of a metal fromthe group consisting of nickel and cobalt.

2. A solution for coating zinc or its alloys comprising zinc ions as itsmajor cation constituent, the negative ions of phosphoric acid as itsmajor anion constituent, the cations of a metal of the group consistingof nickel and cobalt as a minor cation constituent, and nitrate anionsas a minor anion constituent.

3. A balanced phosphatic solution for coating zinc or its alloyscontaining zinc cations and cations of a metal from the groupconsisting'of nickel and cobalt, the ratio of zinc ions to the ions fromsaid group being between approximately 100 and 2; and containing nitrateanions, the ratio of total cations to nitrate anions being betweenapproximately 20 and 2.5.

The sur- 4. The method of coating zinc or its alloys '7 tionconstituent, ions of a metal from the group which consists of nickel andcobalt as a minor cation constituent, and nitrate anions as a minoranion constituent.

6. The method of coating zinc or its alloys which consists in treatingit with a balanced phosphatic solution containing zinc cations andcations of a metal from the group consisting of 7. An article having asurface of zinc or zinc alloy and bearing thereon an adherent, insolublephosphatic coating resulting fromthe treatment of claim a.

8. An article having a surface of zinc or Zinc alloy and bearing thereonan adherent, insoluble phosphatic coating resulting from the treatmentof claim 5.

9. An'article having a surface of zinc or zinc 1 alloy and bearingthereon an adherent, insoluble phos'phatic coating resulting from thetreatment of claim 6.

GERALD C. ROMIG.

